I believe you should always respond whether you feel it is a "low ball" offer or not. I have had people send me really low offers, then we ended up meeting in the middle and making a deal.
I think it's rude to just not even respond to the PM at all.
I agree with this... even if it is an insulting offer its the respectful thing to do to at least give them a response. Never know, they may have been just giving it a shot to buy cheaper and are willing to pay more. If that makes you mad, fine, but at least let them know you aren't willing to go that low and you're not interested in anymore offers from them.Maybe they are jerks but now they think you are, too. IMO, any offer deserves a response and responding to offers should be a requirement of use for any internet classifieds. It takes two people to have a real fight but, as you now know, it only takes one to cause a problem. A simple "Thanks for the offer but I can't go that low" could have saved you from being labeled a scammer and might have lead to an exchange resulting in the sale of the gun.
Being thought of as a jerk has never been a one way street.